The present invention is related to manufacturing facilities and more specifically to exhaust systems in manufacturing facilities.
Some manufacturing facilities house manufacturing equipment and employees in a tightly controlled environment to minimize contamination that can adversely affect the manufacturing process. Semiconductor manufacturing facilities, known as cleanrooms, are an example of such facilities, although the present invention is not limited to semiconductor manufacturing facilities.
Operation and activities that occur during the production of products may produce contaminants that can adversely affect the purity of the air in the facility. For example, the operation of the semiconductor equipment produces noxious pollutants in the process exhaust. The operators themselves also consume oxygen and produce carbon dioxide and other gases that must be removed from the facility.
To preserve the purity of the air in the manufacturing facility, thereby avoiding contamination of the products produced therein, and to protect the health of the workers in the facility, contaminated air must be exhausted from the facility to ambient air outside the facility. When air in a tightly controlled facility is exhausted, costly make-up air must be reintroduced into the facility to maintain sufficient air pressure in the facility. If the air pressure of the facility is not maintained at a higher pressure than the outside air, unpurified outside air will enter the semiconductor fabrication plant through holes and cracks in the facility that lead to the ambient air outside the facility. Thus, as air in the plant and process exhaust is exhausted, make-up air from outside the plant must be highly purified and then blown into the plant to maintain the pressure. Supplying the make-up air incurs an expense which must be borne, including the cost of energy and maintenance costs associated with both the filtration equipment and the blowers.
Because the exhaust from a semiconductor fabrication plant tends to include noxious elements, process exhaust must be released from the plant into the atmosphere at heights determined to be environmentally prudent. However, plants built with taller stacks tend to be aesthetically unappealing and may exceed maximum height ceilings imposed by local governments. To ensure that exhaust from short stacks reaches a height that is higher than the top of the stack, the exhaust is released at a sufficiently high velocity to ensure the exhaust reaches the required height before disseminating into the ambient air outside of the plant. The velocity of the exhaust is based upon the flow of the exhaust and the cross-sectional area of the stack. Thus, it is possible to size the cross-sectional area of the stack to ensure a velocity of the exhaust that can ensure the exhaust reaches a required height under normal operating conditions.
Semiconductor manufacturing plants and other types of manufacturing plants are frequently built in stages. For example, the plant may be built one quarter at a time. In addition, portions built may not be fully operational for various reasons. However, conventional stacks the plant will use for exhaust purposes are built to accommodate the plant when it is fully built and operational. Thus, the cross-sectional area of the stacks are sized to ensure a velocity to allow the exhaust from the plant to reach the desired height only when the plant is fully built and operational. During periods in which the plant is not fully built and operational, the flow of exhaust is less than it will be when the plant is fully built and operational. Thus, the cross-sectional area of the stacks is too large for the flow of the exhaust to allow the exhaust to reach the required height.
To ensure the flow of exhaust reaches the desired height, the air flow of the exhaust may be increased. There are two methods traditionally used to increase the airflow through the stack during periods when the plant is not fully built or operational. One method increases the flow of filtered air through the semiconductor-fabrication plant by employing more blowers, running existing blowers at a higher speed or both. The increased air flow results in increased flow of the exhaust release to maintain the ultimate height of the exhaust. However, because greater volumes of air are filtered and blown, this method increases the costs of supplying make up air and increases the energy costs of the blowers beyond what is necessary to remove the exhaust from the manufacturing facility.
A lower cost arrangement for increasing the flow of exhaust when the facility is not fully built or operational is referred to as induction. Using induction, blowers blow outside air directly into the exhaust stream itself to increase the flow of exhaust. Induction reduces the expense associated with increasing the flow of air because the air from the induction blowers that is blown into the exhaust stack does not need to be as highly purified as the make-up air blown into the manufacturing facility. Nevertheless, the induction blowers increase energy costs and because the induction blowers require maintenance, their use increase maintenance expenditures beyond those necessary for removing exhaust from the facility.
Another weakness of both methods described above is their inability to sense and respond automatically to changes in air flow that occur during day-to-day operations of the fabrication plant. The background art does contain a solution to this problem, using an apparatus for maintaining air flow in a work chamber at a constant velocity by sensing the velocity of the exhaust and adjusting the speed of a blower to compensate (Gray, U.S. Pat. No. 5,356,334, issued Oct. 18, 1994). However, because the Gray apparatus depends upon regulation of air flow by a blower means for moving air, a system or method for maintaining high velocity exhaust release that incorporated the Gray apparatus would still be associated with the energy and maintenance and cost inefficiencies of increased air flow described using one of the two methods described above.
What is needed is a method and apparatus for maintaining exhaust release height above a threshold that senses and responds automatically to changes in air flow that does not rely on increasing the flow of the exhaust.
Velocity of exhaust is maintained above a threshold velocity by an automatic detection and control system that manipulates the cross-sectional area of an opening through which the exhaust is released. The automatic detection and control system ensures that, despite changes in air flow generated by an upstream process, the released exhaust and any noxious elements in it are discharged at a constant velocity to help ensure the exhaust disseminates into the atmosphere at an environmentally safe height.